Last Updated 1 year by Lukas
In this article I would like to cover ways to improve an existing solar system installation. I’ll be doing this using a system installed in my own home. This means that I can give you tested and practical experience of tuning the system. You can then judge for yourself if it makes sense to try to replicate similar changes. The original description of Passive House systems was covered in: Introduction, Solar System description, Recuperation and heat pump system description and finally inside real consumption numbers posts.
What are we going to discuss?
- What has been changed from default settings inside RCT Power Storage DC 10 setup
- What has been changed against original installation of solar system delivered from developer company
- We will also evaluate these changes together and try to assess if it makes any sense
Changes inside inverter default settings
Please read your inverter manual before making any changes so you know what you are doing. RCT Power Storage DC manuals are available here. RCT inverters are configured using the RCT Power app and an Android / iOS device.
Device settings
- Make sure you properly filled in plant peak power
- Ensure that your external power reduction is properly filled in. This limits the maximum power that can be fed into the grid. This is something you agreed with the distributor when you connected your solar power system. If you exceed the agreed value, you may be fined.
- The RCT has two string inputs and a MPP (Maximum Power Point) tracker for each. It works up to 800V (maximum peak DC voltage per string is 1000V). If enabled, every 15 minutes RCT will try to calculate the best voltage for the string to draw the maximum possible current. In other words, it maximises the power from the panels. Please note that this does NOT replace the optimisers installed on the panels. It just helps to get a little more out of the panels (even with optimisers). Be sure to test it and compare it to the previous state, this feature may also reduce efficiency, hence the need for testing and evaluation.
Battery settings
- Please double check with RCT manual ! (As properties may change)
- Currently allowed LiFePo4 usable capacity is 90% (from 7% SOC up to 97% SOC)
- If you want to disable the RCT prediction feature, which controls the charging/discharging of the battery to the grid based on the expected weather. You must select Constant SOC target selection. This feature is not very useful in the Czech Republic as you want to capture as much energy as possible.
- Depending on the number of battery cells, you can go up to 9-10kW of discharge power. In my case, with an 11.5 kWh battery (currently maximum supported size), I chose 9 kW. At 10kW my RCT failed with a battery over_current error message and restarted itself (this seems to be caused by the latest FW as it previously worked at 10kW).
Switching on conditions
- Switching on time after any grid fault – 300 seconds. To make sure RCT waits a while before it tries to go into island mode
- Max additional random start-up time after any main power grid failure – 600 seconds. To ensure that if the RCT doesn’t switch to island mode and fails (e.g. it’s overloaded). It will add random time and wait, this prevents RCT cycling immediately after failure (which is not very good for electronics).
Control update
- Make sure your RCT is up to date. Be careful with upgrading FW, you can only do it if there is enough power from Sun. Please check inside manual, before attempting to update RCT inverter !
- Make sure you use the correct standard for your country. This will set default parameters for how RCT will behave. For example, how it reduces power, how it reacts to under/over voltage or under/over frequency. These parameters are provided by your local distributor and are mandatory. If you run the inverter with incorrect settings and you are connected to / exporting to an external grid. You may be fined!
RCT monitoring aplication
- RCT inverter is also able to show you overview of current production / consumption with many detailed parameters
- There is also available data log with historical measured values (possible export into CSV) and with error codes log (These can be found inside RCT manual)
Changes against original system
- Original system provided with delivered house from developer
- Power 5,4 kWp (one string, all panels with optimizers)
- Inverter RCT Power Storage DC 6.0 kW
- LiFePo4 battery pack 9,6 kWh
- Storage into water controlled through PLC and Solid State Relays
- Allowed export into outside grid 5 kW
- Maximum temparature of 425 litres water tank 70 degrees
- Currently used system
- Power 14 kWp (two strings, all panels with optimizers)
- Inverter RCT Power Storage DC 10.0 kW
- LiFePo4 bettery pack 11,5 kWh
- Storage into water controlled through PLC and Solid State Relays
- Additional 65 litres water heater
- Allowed export into outside grid 10 kW
- Maximum temperature of 425 litres water tank 75 degrees
- changed heating temperature of heat pump from 52 into 37 during cold days and 27 during sunny days. To maximize power stored into water from solar system.
Does it all really make any sense?
You may ask yourself whether it’s a wise decision to create such a system for a single house. The total cost of the system is a nice CZK 1 000 000,- (price of the original system from the developer + changes to the system). You can apply for a bonus from the Czech government (up to CZK 250,000). Let’s assume that the final price of such a system, which you will have to pay out of your own pocket, will be around CZK 750 000. Let us also assume that the lifetime of the inverter is 10 years, the battery 10 years and the solar panels 30 years. And we will sell extra electricity for SPOT price (electricity we cannot use or store in the house). The price of a new inverter is about CZK 100 000,-, the price of a new battery is about CZK 300 000,- (we assume that the price of the same battery with the same capacity will decrease over time). Let’s assume an annual production of about 14 MWh and a self-consumption of 70%. Now we will try to make a calculation.
- Yearly production from Solar System – approximately 14 MWh
- Self-consumption of the house – 14 * 0,7 = 9 800 W (price 150 * 9,8 = EUR 1 470,- )
- Currently (15.04.2023) inside Czech Republic you would pay for this around CZK 55 000,- per year
- Export into the grid – 14 * 0,3 = 4 200 W (price 150 * 4,2 = EUR 630,-)
- Currently inside Czech Republic you would sell this for CZK 15 000,- per year
- Consumption from outside grid 3,5 MWh
- Currently inside Czech Republic you would pay for this around CZK 25 000,- per year
- Current price of electricity (SPOT) as of 02/04/2023 – 150 EUR / MWh
- In total savings per year are
- CZK 55 000,- (You would have to otherwise pay for 9 800W consumed from grid)
- CZK 15 000,- (These are money you are provided for selling electricity)
- In total CZK 70 000,- per year saved
- Given the price of the system (and without using it for electric car charging) it will take at least 10 years to get your money back
- The magic begins when you can take bigger perspective (long term point of view) and synergy from the system like selling electricity, charging your e-car. In such case return time can decrease by 2-3 years.
- You can also help a lot to calculation if you put in inflation rate and some assumed electricity growth rate. No-one can really predict that long into future. But many providers are doing it as it looks nice and it can easily persuade people to buy such system.
- Self-consumption of the house – 14 * 0,7 = 9 800 W (price 150 * 9,8 = EUR 1 470,- )
Of course, there are many variables in the above calculation. Please keep in mind that if you are not able to sell extra electricity, you will consume as much electricity yourself as possible (requires battery, which is very expensive). Or you do not own an electric, plug-in hybrid car to store electricity and use it synergistically. You may be wasting a lot of money that will never come back. If we take a step back, it’s unlikely that electricity prices will go back to 20 EUR / MWh. The main reason for this is electromobility and the huge investments needed in the electricity grid system. In other words, these investments have to be paid by someone. And most likely it will be through electricity prices. Furthermore, if you look at the prices of charging electric cars. It seems the most efficient is to charge them at home from your solar system.
